Electric heater



Sept. 8, 1936. c, c. ABBOTT .2,053,933

ELECTRIC HEATER Filed Aug. 12, i955 Invent ow:

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Patented Sept. 8, 1936 UNITED STATES ELECTRIC HEATER Charles C. Abbott,Pittsfield, Mass., assignor to General Electric Company, a corporationof New York Application August 12, 1933, Serial No. 684,832

4 Claims.

My invention relates to electric heaters, and it has for its object theprovision of a simple, efiicient and inexpensive heater of this type andmethod of making it.

Although not limited thereto, my invention is particularly useful inconnection with relatively low temperature electric heaters of thesheathed type wherein a resistance conductor is enclosed by a metallicsheath.

In carrying out my invention in one form thereof, a resistance conductoris wound upon an electrically insulating core, which preferably will beformed of a yielding fibrous insulating material, such as asbestos. Thiscore is reinforced by means of a metallic wire or thread arrangedcentrally of the core. The core may be made by covering the wire withasbestos, roving in any suitable manner.

The resistance Conductor and core are enclosed by an elongated metallicsheath, and preferably will be arranged substantially centrally of thesheath. Interposed between the resistance conductor and the sheath is ahighly compacted layer of yielding, fibrous, electrically insulatingmaterial, such as asbestos. Suitable terminals are electricallyconnected with the ends of the resistance conductor and in one form ofmy invention, these terminals project from the sheath. In another formof my invention, the terminals are enclosed by the sheath and suitableexternal leads enter the ends of the sheath where they are electricallyconnected with the terminals. In this form, it is preferable to reducethe sheath materially at the ends so that the sheath fits tightly aboutthe leads. The leads in this case are permanently attached to theterminals.

In making an electric heater in accordance with my invention, a suitableresistance conductor is wound upon a core formed as described above, andthen suitable terminals are connected to the ends of the resistanceconductor. This may be accomplished by compressing metallic tubularterminal members upon the end portions of the core in electrical Contactwith the end turns of the resistance conductor. These terminals may becompressed by swaging them upon the ocre.

Then suitable leads may be attached to the terminals, if it is desiredto use permanently attached leads, and after this the resistanceconductor and the core and also the terminals are covered with a `layerof asbestos which may be in the form of a roving. Then the assembly isinserted in a metallic sheath which is subsequently reduced in diameterso as to highly compact the asbestos insulating material. The sheath maybe reduced by swaging or rolling.

For a more complete understanding of my invention, reference should behad to the accompanying drawing in which Fig. 1 is a fragmentary view inelevation illustrating a resistance conductor wound upon a flexiblesupporting core arranged in accordance with my invention, and alsoshowing a terminal connector member used with my electric heater,portions of the core and terminal being shown in section so as toillustrate Certain structural details; Fig. 2 is a fragmentary View,partly in section, illustrating the manner in which the terminals areconnected to the resistance conductor and also the manner in which leadsare connected to the terminals, all arranged in accordance with myinvention; Fig. 3 is a fragmentary view, mainly in section and taken onan enlarged scale, of a finished electric heater made in accordance withmy invention; Fig. 4 is a complete elevation on a reduced scale of thefinished electric heater of Fig. 3; and Fig. 5 is an elevation, partlyin section, illustrating a nished electric heater embodying a modifiedform of my invention.

Referring to Figs. 1-4 inclusive, I have shown my invention inConnection with an electric heater comprising a conductor I0 mountedupon a suitable supporting core II. The resistance conductor Il) may bemade of any suitable material, but preferably will be made of anickel-chromium alloy. Preferably, the supporting core I I will be madeof a suitable electrically insulating, exible and slightly yieldingmaterial, such as asbestos. And preferably, the asbestos will bereinforced by a ne metallic wire or thread I2 arranged centrally ofthecore. The core I I may be conveniently formed by applying asbestosroving to the wire I2 so asvto cover the wire with a substantiallyuniform layer of the asbestos. The thread I2, as shown, preferably willbe formed by twisting together a plurality of relatively ne wires.

When a very large number of heating elements are to be made, it isconvenient to wind the resistance conductor III continuously upon arelatively long length of the asbestos core I I, after which the coreand the conductor wound upon it may be cut into portions of the requiredlength vfor the various heating units.

The provision of the heat resistant, flexible core II is an importantfeature of my invention. It will be appreciated that in an electricheater adapted for low temperature work, it is desirable to use a veryfine resistance conductor in order to secure the required resistance. Inorder to use satisfactorily a very ne resistance conductor, I haveprovided the conductor with the flexible heat resistant core I I whichserves both to support the turns of the resistance conductor and toretain the various turns in their proper spaced relation when wound. Itwill be observed that since the conductor is provided with a supportwhich is slightly yielding, the conductor as it 1s being wound willslightly depress the outer surface of 2 aotaaaa the support so that eachturn of the conductor will form its own recess or groove. By reason ofthis construction any displacement of the turns is prevented. It will beunderstood, however, that the asbestos supporting member II issufficiently hard to provide a comparatively strong and firm support forthe conductor I0, it being yielding merely to the extent of permittingthe resistance conductor to slightly depress the outer surface. In otherwords, the supporting member II will be one possessing substantialmechanical strength. v

After the conductor has been wound upon its core, suitable terminals I3are connected to the end portions of the resistance conductor. Theterminals I3 are connected to the resistance conductor by means ofsuitable terminal connector members I Il. 'Ihe connector members Id areof tubular form and, as shown, are provided with a stepped bore; thisbore has a relatively large portion Ia which serves to receive the endportion of the core with the resistance conductor wound upon it and aportion Ib of somewhat smaller diameter which receives the end of theterminal I 3.

In assembling each terminal I3 with the heater, the end portion of thecore with the resistance element wound upon it that is to be attached tothe terminal is inserted in the bore Ilia, in a connector member I6.Then the connector member is reduced in diameter so that the conductorwill be pressed into good mechanical and electrical contact with the endturns of the coiled resistance unit. This reducing step may beaccomplished by rolling or swaging the connector. Preferably, before thecore is attached to the terminal,

the end of the conductor will be wound back in the opposite direction toform three or four turns Ia upon the end turns already Wound, as shownin Fig. 1. It has been found that this insures a very good electricalcontact between the resistance conductor and the connector after theconnector has been applied, and furthermore, that this arrangementobviates danger of the connector pulling away from the core. It is alsopreferable to swage the end portion of the core somewhat before it isapplied to the connector so as to size it to fit the bore portion Ma, itbeing understood that the portion Ia before it is swaged has a diametersomewhat smaller than the diameter of the core.

After this, the terminal I3 is inserted in the bore Mb provided for itin the connector, and this portion of the connector is swaged by meansof square dies so as to firmly connect the terminal to the connector. Itwill be understood that when the portion Ida was reduced upon the core,the diameter of the bore Ib will have been reduced somewhat at the sametime. Because of this, the original diameter of the bore portion Ib willbe chosen so that after the connector is applied to the core, the boreIb will have a diameter approximately equal to the diameter of theterminal I3. Then the terminal may be inserted in the portion I 4b whichwill be swaged upon the terminal, as described.

It will be understood, of course, that terminals I3 will be connected toboth ends of the resistance heating element by connectors Ill in themanner just described.

It will be observed that when the connector III is reduced upon its endof the core I l, the asbestos forming the end portion of the core willbe squeezed into the smaller bore Ilb as clearly shown in Fig. 2. Thisis important because this squeezed portion completely and effectivelyeleotrically insulates the connector Il and terminal I3 from themetallic wire IZ.

Suitable leads I5 are then connected to the free ends of the terminalsI3. For this purpose, lead connector members I are provided. Theconnector members I6 are similar to the connector members III. Thus,each member I6 is provided with a bore Ia of relatively small diameterand with a second bore IBI) of relatively large diameter aligned withit. The diameters of these two bores before the connector is applied tothe terminal and the lead are substantially uniform throughout theirlengths, as are the bores IIIa and Ibof the connector III. The connectorIS is applied to the lead I5 by baring an end portion of the lead of itsinsulation, as clearly shown in Fig. 2, and then inserting the baredconductor Ia into the bore ISb. The connector is then reduced indiameter by swaging or rolling so as to compress it tightly upon thebared conductor. The eifect of this operation is to leave a head Ib onthe end of the conductor, as clearly shown in Fig. 2. During thisreducing operation, the bore Ia is reduced somewhat, but its originaldiameter is chosen so that after the reducing process it will beapproximately the same size as the terminal I3. The terminal I3 is theninserted in the bore Ita which is swaged by means of square dies so asto reduce its diameter sufciently to eiect a very good mechanical andelectrical connection between the terminal and the connector.

It will be understood that the leads I5 for both ends of the heater willbe connected in this same manner.

The resistance element I0 and the core Il on which it is mounted arecompletely covered throughout their entire length by a layer ofinsulating material Il which is preferably formed of a fibrous yieldablesubstance, such as asbestos. This asbestos may be conveniently appliedin the form of a roving. It is also preferable to cover the terminalconnectors Id and the terminals I3 throughout substantially their entirelengths by means of the asbestos roving I1. The lead connectors I6 andthe adjacent end portions of the leads are covered by an insulatinglayer I8, which may be formed of any suitable substance, but whichpreferably will be made of asbestos tape impregnated with a suitableelectrically iri-1 sulating compound, such as a resin of the alkyd type.

When a large number of heating elements are to be made, the resistanceconductor and the terminals I3 may be conveniently covered with theinsulating layer I'I in the following manner. After the resistanceconductor I0 has been wound continuously upon a relatively long core IIand has been cut apart to the desired lengths, as previously described,the separate resistance sections are connected together by means of terminal conductors having double the length of the terminals I3. Theseconductors will be secured to the ends of the resistance sections bymeans of the connectors I, and are subsequently used to form theterminals I3. The desired number of the resistance sections are securedtogether in this manner so as to form a relatively long chain which maybe Wound upon a suitable collecting reel. It will be understood that theterminal conductors connecting the resistance sections are sufficientlyilexible to permit the chain to be wound upon the reel. The resistanceelements thus connected together may be run through a suitableinsulating machine (not shown) for applying the asbestos roving l1 tothe chain so as to completely cover the resistance sections, theconnectors and the terminal conductor lengths between the sections. 'Iheresistance sections are then separated by cutting through the terminalconductors midway between the connectors. This operation, of course,divides the chain into a number of separate resistance sections, to theends of which are'secured theseveral half sections of the terminalconductors that,-

previously connected the sections together. These half sections form theterminals I3. After this, the insulation I1 on the free ends of theterminals I3 is stripped back to provide for connection of the terminalswith the leads I5 by means of the connectors I6. After the leads havebeen attached, the insulating layers I8 will be applied to theconnectors I6 and to the adjacent ends of the leads.

The entire assembly including the core II, the resistance conductor I0,the terminals I3, the leads I5 and the insulating layers I1 and I8, isthen threaded through a suitable metallic sheath 2D. The sheath willhave such a diameter that the assembly will have a sliding t with thesheath.

Then the sheath 20 is reduced in diameter so as to highly compact theinsulating layers I'I and I8 about the resistance element, itssupporting core, the terminals and the connectors I4 and I6.

The sheath may be reduced in diameter by swaging or rolling. The endportions 20a. of the sheath are reduced to a smaller diameter than theintermediate portions of the sheath so that these end portions t tightlyon the leads. This arrangement prevents the entrance of moisture andlike uids into the sheath. The asbestos tape I8 impregnated with thealkyd resin assists greatly in rendering the heater moisture proof,

The sheath will be reduced considerably in diameter so as to thoroughlycompact the insulating layers I1 and I8. Thus, for example, the sheath,if it had a diameter of It will also be observed that the turns oftheconductor cannot become displaced and short circuit each other sothat the finished heating element can be bentinto suitable desiredshapes.

In Fig. 5, an electric heater embodying a modied form of my invention isshown. In this heater, the resistance conductor 30 is wound upon a core3I which is arranged substantially as is the core II of Figs. 1-4inclusive. Thus, the core 3l is formed of asbestos reenforced by ametallic twisted wire or thread (not shown). The resistance conductor 30is electrically connected at its ends with terminals 33 through themedium of connector members 34, and` arranged as are the correspondingelements of Figs. 1 to 4.

The resistance conductor and the attached terminals are`then completelycovered with a suitable layer 35 of insulating material, which will besimilar to the insulating material Il, that is, it will Vbe formed ofasbestos roving. Theassembly is inserted in a metallic sheath 35, whichwill have a diameter substantially equal to the diameter of the assemblyand a length such that the ends of the terminals project from thesheath. Then the diameter of the sheath is reduced as by rolling orswaging so as to compress the asbestos insulation tightly about theresistance element, core and terminals.

While I have shown particular embodiments of my invention, it will beunderstood, of course, that I do not wish to be limited thereto sincemany modications may be made, and I, therefore, contemplate by theappended claims to cover any such modications as fall within the truespirit and scope of my invention.

What I claim asnew and desire to secure by Letters Patent of the UnitedStates, is:

1. An electric heater comprising a core formed of a yieldingelectricallyinsulating material, a resistance conductor wound upon said core, theends of said conductor being wound back in a reverse direction on theend turns of the conductor on said core, and tubular terminal memberscompressed on said end turns and on said re-l Vversely wound ends so asto provide a good electrical and mechanical connection between saidterminal members and said resistance conductor.

2. An electric heater comprising a metallic thread, a layer of fibrousyielding electrically insulating material covering said metallic thread,a resistance conductor wound upon said layer of insulating material,tubular terminals embracing the end turns of said resistance conductor,and compressing said electrically insulating material about the ends oflsaid metallic thread so as to insulate said thread from said terminals,a second layer of brous yielding electrically insulating materialcovering said resistance conductor and a metallic sheath encasing saidsecond layer of insulating material and compressed thereon so as tohighly compact said layers of insulating maerial.

3. An electric heater comprising a yielding flexible core formed ofelectrically insulating material reenforced by a metallic threadarranged substantially centrally thereof, a resistance conductor woundupon said core, and tubular ter-- minal members compressed upon the endsof said core in electrical contact with the end turns of said resistanceconductor arranged to compress the ends of said insulating material ofsaid core over and about the ends of said metallic thread.

4. -The method of making an electric heater which comprises forming acore of flexible yielding electrically insulating material by covering ametallic thread with a layer of said material, winding a resistanceconductor upon said core, compressing tubular terminal members on theend portions of said core so as to mechanically and electrically connectsaid members with said resistance conductor and to compress the endportions of said insulating material within said tubular members overthe ends of said metallic thread to thereby electrically insulate saidthread from said terminals, covering said resistance conductor and corewith a layer of flexible yielding electrically insulating material,inserting said core with said layer of insulating material thereon intoa metallic sheath and elongating and reducing said sheath so as tocompact said layer and core of insulating material. i

' CHARLES C. ABBO'IT.

